http://www.psycharts.com/illusions.htm We think we perceive objective reality, but perception is always altered by invisible biases. We are never free of the state-dependent bias, and so we are continually taken in by an illusion.

http://wisebytes.net/illusions/ This site shows a range of common visual illusions along with a brief explanation of why they occur. It was originally prepared to show at a children's primary school as part of their study of sensory processing.

http://facstaff.uww.edu/mccreadd/ A relatively new theory is offered for the classic illusion that the moon's constant subtended visual angle of half a degree appears larger for the horizon moon than for the zenith moon.